The present invention relates to a method and system that can achieve simultaneous attitude, momentum, and orbit control using gimbaled and throttled thrusters.
For a spacecraft to perform its mission generally requires control of its in-orbit position relative to the Earth. For example, the orbit of a geosynchronous communications spacecraft is controlled so that the spacecraft remains stationed above the equator at a fixed earth longitude within small tolerances in the north/south and east/west direction. This position control is necessary to allow access to the satellite using fixed-direction ground antennas and to prevent both signal and physical interference with nearby spacecraft.
The orbit control is accomplished by performing periodic orbital correction maneuvers, where thrusters are fired to change the spacecraft""s velocity. During maneuvers, the attitude of the spacecraft is controlled so that it remains correctly pointed relative to the Earth and the momentum stored by actuators such as reaction or momentum wheels is adjusted. The fuel efficiency of the system is determined not only by the thruster technology used, but also by how the attitude and momentum control and the velocity change tasks are accomplished. Systems are desired that are as efficient as possible, thereby reducing the propellant required and allowing the payload mass of the spacecraft to be increased.
Various types of thrusters may be used to produce an orbital velocity change. For example, Hall Current Thrusters that accelerate Xenon ions through an electric field to generate thrust may be utilized. Alternatively, Arcjet thrusters that heat hydrazine decomposition products using an electric arc may be employed. Generally, thruster technology is selected that provides the highest propellant efficiency and is compatible with the spacecraft design. Whichever types of thrusters are utilized, they typically are mounted on a spacecraft or on a boom or other structure attached to a spacecraft to provide components of thrust in at least the east/west and north/south directions.
Early prior-art systems utilized thrusters whose thrust directions are fixed in the spacecraft body frame. Attitude and momentum control is provided during velocity change maneuvers by firing separate attitude control thrusters, typically hydrazine thrusters known as rocket engine assemblies or REAs. Because the control thrusters are generally much less fuel efficient than the velocity change thrusters, the overall system fuel efficiency is reduced using this approach.
Recently, improved methods have been introduced that mount the velocity change thrusters on gimbaled platforms. The gimbaling allows the orientation of each thruster""s thrust vector to be varied. By commanding changes in the gimbal angles and possibly the thrust levels of the velocity change thrusters, attitude and momentum control can be provided without firing separate control thrusters.
These systems provide improved fuel efficiency, but still have several drawbacks. First of all, the direction of the applied velocity change is not actively controlled. For example, firing thrusters to produce a velocity change in the north/south direction may also produce an unwanted change in the east/west direction. A separate maneuver is then required that applies an east/west thrust impulse to correct the orbit for the effects of the unwanted east/west impulse. These additional correction maneuvers use fuel, which must be carried at the expense of added payload mass. In addition, known systems do not provide the capability to control the thruster orientations to be close to reference cant angles while a maneuver is in progress. Known systems simply command whatever changes in gimbal angles or thrust levels are necessary to achieve the required torque.
To improve fuel efficiency over that possible using known systems it may be desired to control the cant angles of the velocity change thrusters relative to the solar array. Such cant angles may be determined to maximize the thrust in the north/south direction or to achieve some other objective such as to minimize solar array plume impingement. Systems are needed that can drive the thruster cant angles as close as possible to desired reference cant angles while simultaneously providing attitude and momentum control and achieving the commanded velocity change.
Among advantages of the present invention are that it can address the above-described limitations as well as other limitations of the known systems.
The invention uses gimbaled and throttled thrusters to produce a commanded change to the spacecraft""s orbital velocity, while simultaneously providing attitude and momentum control. A key advantage of the invention is that it can provide active control of the magnitude and direction of the applied impulse or velocity change. In addition, it can control the thruster cant angles and thrust levels to be close to reference cant angles and thrust levels while a maneuver is in progress. Both of these features allow the present invention to produce significant fuel savings compared to prior-art systems. The ability to control the thrust vector direction can be used to eliminate unwanted east/west coupling during north/south maneuvers, or to introduce east/west coupling intentionally when it will have a beneficial effect on the orbit. In addition to reducing the orbit control fuel required, this capability can improve the accuracy of the orbit position control making it easier to co-locate several satellites in a single geosynchronous orbital slot. The ability to control the cant angles of the thrusters can be used to improve fuel efficiency by maximizing thrust in the desired velocity change direction or to achieve other objectives as described below.
In providing the above and/or other advantages, the present invention relates to a method for controlling spacecraft velocity. A thruster torque gimbal angle command is calculated to provide a desired thruster torque demand command including a desired momentum adjust torque and a desired attitude control torque. A velocity change error between a commanded velocity change and an actual velocity change, a commanded force vector based on the error to cause the actual velocity change to approach a commanded velocity change, and force gimbal angle commands and force thrust commands to achieve the commanded force vector are calculated; and/or an error between gimbal angles and thrust levels and reference gimbal angles and thrust levels, reference gimbal angle commands and reference thrust commands to drive actual gimbal angles and thrust levels toward reference gimbal angles and reference thrust levels are calculated. A total gimbal angle command is calculated by adding the torque gimbal command with at least one of the force gimbal angle command and the reference gimbal angle command. A total thrust command is calculated including at least one of the force thrust command and the reference thrust command. The thrusters are gimbaled to the total gimbal angle command and a level of thrust produced by the thrusters is varied according to the total thrust command.
Additionally, the present invention concerns an attitude control system for a spacecraft that includes a north side, a south side, an east side, a west side, an earth-facing side, and an anti-earth facing side. The attitude control system includes at least one pair of gimbaled orbit adjust thrusters located on the north side and/or the south side of the spacecraft, the thrusters gimbaling in two directions. A controller gimbals the thrusters and operates the thrusters at a thrust level to achieve a desired velocity change and to compensate for deviations in the desired pointing of the spacecraft as a result of operation of the thrusters. The controller calculates a thruster torque gimbal angle command to provide a desired thruster torque demand command including a desired momentum adjust torque and a desired attitude control torque. The controller also calculates a velocity change error between a commanded velocity change and an actual velocity change, a commanded force vector based on the error to cause the actual velocity change to approach a commanded velocity change, and force gimbal angle commands and force thrust commands to achieve the commanded force vector are calculated; and/or the controller calculates an error between gimbal angles and thrust levels and reference gimbal angles and thrust levels, reference gimbal angle commands and reference thrust commands to drive actual gimbal angles and thrust levels toward reference gimbal angles and reference thrust levels. Furthermore, the controller calculates a total gimbal angle command by adding the torque gimbal command with at least one of the force gimbal angle command and the reference gimbal angle command. Additionally, the controller calculates a total thrust command including at least one of the force thrust command and the reference thrust command. The controller gimbals the thrusters to the total gimbal angle command and commands a level of thrust produced by the thrusters according to the total thrust command.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from a review of the following detailed description. The detailed description shows and describes preferred embodiments of the present invention, simply by way of illustration of the best mode contemplated of carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the drawings and description are illustrative in nature and not restrictive.